Polymerization generally involves polymerization of one or more monomers to make a polymeric product. The polymerization reaction can be carried out using a wide variety of reactors, catalysts, and a wide variety of monomer feeds. Often, liquids, diluents or solvents are used in these polymerization reaction processes for various reasons such as to increase the efficiency of the polymerization reaction and recovery of polymer product.
In many polymerization processes for the production of polymer, a polymerization effluent is formed which is a slurry of particulate polymer solids suspended in a liquid medium, ordinarily the reaction diluent and unreacted monomers. A typical example of such a process in a loop reactor is disclosed in Hogan and Bank's U.S. Pat. No. 2,825,721, the disclosure of which is incorporated herein by reference. Variations and improvements of such a process in a loop reactor are described in U.S. Pat. Nos. 2,915,513, 3,152,872, 3,293,000, 3,324,093, 3,625,658, 3,816,383, 3,858,943, 3,956,061, 4,007,321, 4,121,029, 4,199,546, 4,372,758, 4,395,523, 4,424,341, 4,461,889, 4,501,885, 4,589,957, 4,613,484, 4,632,976, 4,690,804, 4,737,280, 4,794,151, 5,183,866, 5,207,929, 5,292,863, 5,391,656, 5,455,314, 5,565,175, 5,575,979, 5,597,892, 6,204,344, and 6,239,235, the disclosures of which are fully incorporated herein by reference.
Typical examples of such a process in a stirred tank reactor are disclosed in U.S. Pat. Nos. 3,825,524, 4,187,278, and 4,492,787, the disclosures of which is incorporated herein by reference. Variations and improvements of such a process in stirred tank slurry reactor systems are known to those skilled in the art.
In most commercial scale operations, it is desirable to separate the polymer and the liquid medium comprising an inert diluent and unreacted monomers in such a manner that the liquid medium is not exposed to contamination so that the liquid medium can be recycled to the polymerization zone with minimal if any purification. The liquid medium used in slurry polymerization processes is typically a saturated hydrocarbon such as isobutane or hexane. Although such diluents are not reactive in the polymerization process, the operating window (i.e. temperature and pressure) under which the polymerization process may be operated is limited by fouling in the reactor caused by agglomeration of the polymer solids in the slurry or deposition of polymer on the wall of the reactor making it impossible to recover the polymer product.
Within the conventional operating window, a particularly favored technique that has been used heretofore is that disclosed in the Scoggin et al, U.S. Pat. No. 3,152,872, more particularly the embodiment illustrated in conjunction with FIG. 2 of that patent. In such processes the reaction diluent, dissolved monomers, and catalyst are circulated in a loop reactor wherein the pressure of the polymerization reaction is about 100 to 700 psia (689 to 4826 kPa). The produced solid polymer is also circulated in the reactor. A slurry of polymer and the liquid medium is collected in one or more settling legs of the slurry loop reactor from which the slurry is periodically discharged to a flash chamber wherein the mixture is flashed to a low pressure such as about 20 psia (138 kPa). Other preferred methods for recovery of polymer product and recirculation of diluent back in to the polymerization process are shown in U.S. Pat. No. 6,204,344 to Kendrick et al. and U.S. Pat. No. 6,239,235 to Hottovy et al. The continuous withdrawal of slurry from the reactor instead of the intermittent withdrawal method permits operation of the reactors at a higher solids content which in turn leads to economically desirable higher polymer production rate for the same reactor volume. These methods also reduce the cost of diluent recovery and recirculation by utilizing a two-stage flash process wherein the first flash is performed at a pressure and temperature permitting the diluent to be reliquefied by heat exchange without the need for compression. These systems are limited in that the maximum comonomer incorporation or minimum polymer product density is limited by fouling of the polymer product in the reactor.
An example of a polymerization process that incorporates the use of a diluent other than a saturated hydrocarbon is shown in U.S. Pat. No. 3,470,143 to Schrage et al. Specifically, the Schrage patent discloses a laboratory scale polymerization reaction that incorporates the use of a fluorinated organic carbon compound as the diluent. Schrage discloses preparation of a boiling-xylene soluble polymer in a slurry which comprises polymerizing at least one ethylenically unsaturated hydrocarbon monomer to an amorphous elastomer in a reaction zone which comprises employing as a polymerization medium a fluorinated organic carbon compound.
EP 1 323 746 shows loading of biscyclopentadienyl catalyst onto a silica support in perfluorooctane and thereafter the prepolymerization of ethylene at room temperature.
U.S. Pat. No. 5,624,878 discloses the polymerization using “constrained geometry metal complexes” of titanium and zirconium.
There are always needs for improved polymerization processes. In particular, it would be advantageous in slurry polymerization processes using chromium catalysts to provide expanded operating limits in terms of pressures and temperatures and expanded product slates including lower density products than previously made in such processes and the ability to increase comonomer incorporation into a polymer chain at constant comonomer input rate. It would be further advantageous to improve such processes by providing more efficient separation of polymer product from the diluent.